1. Field of the Invention
This invention relates to a heat exchanger using hollow fibers made of an organic polymer as the heat transfer tubes and capable of efficiently warming or cooling various fluids including liquids such as water or blood and gases such as air, oxygen or nitrogen and to a blood oxygenating device furnished with this heat exchanger.
2. Description of the Prior Art
Conventionally, various types of heat exchangers are known as devices for transferring heat from a high-temperature fluid to a low-temperature one. Most typical heat exchangers have a multitubular construction. For use as the material of heat transfer tubes in a heat exchanger of the multitubular type, metals having good heat conductivity are most effective. Among others, stainless steel pipes have been commonly used because of their excellent resistance to corrosion by the fluids involved in heat exchange. An effective method for installing stainless pipes in a heat exchanger is potting with an organic resin, but the large difference in hardness between the stainless steel pipes and the potting material makes it difficult to process the end surfaces of the potting members. That is, the ends of the pipes have exposed sharp edges which, in the treatment of a fluid containing particles as blood cells, tend to destroy those particles. In order to overcome this difficulty, the use of stainless pipes whose tips are covered with soft pipes is under investigation, but no marked improvements have been produced.
A heat exchanger is used as a means for the heat exchange of various fluids. For example, when a blood oxygenator is used to perform an operation on the heart, a heat exchanger is usually added to the blood gas-exchange circuit including the blood oxygenator because of the necessity to adjust the body temperature of the patient to a low level at the beginning of the operation, the necessity to make the temperature of the blood having undergone gas exchange by means of the blood oxygenator almost equal to the body temperature of the patient before returning it to the body of the patient, or the necessity to restore the lowered body temperature of the patient to the normal level after the operation. In medical facilities such as hospitals and the like, this blood gas-exchange circuit is generally assembled by connecting a blood oxygenator with a separate heat exchanger by means of, for example, circuit tubes. However, such an arrangement is disadvantageous in that assemblage of the blood gas-exchange circuit is troublesome to the user, there is a risk of erroneous assemblage of the circuit, and additional space for the circuit is required. Moreover, since the blood oxygenator and the heat exchanger involves two separate stagnation sites of the blood and necessitate circuit tubes to connect them, the priming blood volume required at the initial stage of operation of the circuit is unduly large and the various circuit components must be degassed separately. Thus, such an arrangement is also complicated from the viewpoint of operation.
As means for overcoming these disadvantages, blood oxygenating device comprising a blood oxygenator combined with a heat exchanger to form an integral unit are disclosed, for example, in Japanese Patent Publication No. 2982/'80 and Japanese Patent Laid-Open No. 39854/'82. In these blood oxygenating devices, however, the heat transfer member of the heat exchange section is formed of a metal such as stainless steel having good thermal conductivity. In the case of stainless steel pipes, additional difficulties may be encountered because the metallic debris produced during processing of the pipe ends may remain in the pipes and contaminate the blood and, moreover, stainless steel may be reactive with some components of blood having a complicated composition. Accordingly, there is a continuing demand for a heat exchanger diminishing these difficulties.
On the other hand, a number of blood oxygenators using a hollow-fiber membrane have already been proposed, for example, in U.S. Pat. Nos. 2,972,349, 3,794,468, 4,239,729 and 4,374,802.
In these blood oxygenators, hollow fibers made of a homogenous membrane of gas-permeable material such as silicone or hollow fibers made of a microporous membrane of hydrophobic polymeric material such as polyolefins are used to bring blood into contact with gas through the medium of the hollow-fiber membrane and effect gas exchange therebetween. There are two types of blood oxygenators: the inside perfusion type in which blood is passed through the bores of the hollow fibers while gas is passed on the outside of the hollow fibers and the outside perfusion type in which, conversely, gas is passed through the bores of the hollow fibers while blood is passed on the outside of the hollow fibers.
In most of the conventionally known blood oxygenators, a cylindrical housing is simply packed with a large number of hollow fibers of semipermeable membrane for use in gas exchange in such a way that the hollow fibers are parallel to the axis of the cylindrical housing. However, blood oxygenators of this construction have low gas exchange rate per unit area of hollow-fiber membrane, whether they are of the inside perfusion type or of the outside perfusion type. As an improved form of the outside perfusion type, U.S. Pat. No. 3,794,468 has proposed a blood oxygenator in which hollow tubular conduits of semi-permeable membrane are wound about a hollow, cylindrical core having a large number of pores in the wall and then contained in a housing, and blood is allowed to flow out of the cavity of the core through its pores while gas is passed through the bores of the hollow tubular conduits.
In blood oxygenators of the inside perfusion type in which gas exchange is effected by passing blood through the bores of the hollow fibers while passing gas on the outside of the hollow fibers, channeling of the blood occurs less frequently. However, since the blood flowing through the bores of the hollow fibers moves in a laminar flow, the internal diameter of the hollow fibers needs to be reduced in order to increase the oxygenation rate (i.e., the oxygen transfer rate per unit area of membrane). For this purpose, hollow tubes of semipermeable membrane having an internal diameter of the order of 150-300 .mu.m have been developed for use in blood oxygenators.
Nevertheless, as long as the blood moves in a laminar flow, the oxygenation rate cannot be greatly increased by reducing the internal diameter. Moreover, as the internal diameter becomes smaller, clotting (i.e., blockage of the bore due to the coagulation of blood) may occur more frequently and/or the blood will be more subject to hemolysis due to an increased pressure loss through the oxygenator, thus posing serious problems from a practical point of view. Furthermore, since a blood oxygenator generally uses tens of thousands of hollow fibers of semipermeable membrane made into a bundle or bundles, special consideration must be given so as to distribute the gas uniformly to the external surfaces of each of these numerous hollow fibers. If the gas is not distributed uniformly, the carbon dioxide desorption rate (i.e., the carbon dioxide transfer rate per unit area of membrane) will be reduced. On the other hand, in blood oxygenators of the outside perfusion type in which gas is passed through the bores of the hollow fibers while blood is passed on the outside of the hollow fibers, the gas can be distributed uniformly and the blood can be expected to flow turbulently. However, these oxygenators have the disadvantage of being subject to insufficient oxygenation due to channeling of the blood and/or blood coagulation at the sites of stagnation. Although the blood oxygenator of the aforementioned U.S. Pat. No. 3,794,468 has undergone improvements in this respect, it is still disadvantageous in that the priming blood volume is unduly large, a considerable pressure loss through the oxygenator is caused, and a complicated procedure is required for the manufacture thereof. Thus, it remains desirable to develop a more improved blood oxygenator.